1. Field of the Invention
This invention generally relates to communication systems and, more particularly, to a system and method for adjusting signal enhancement mechanisms on the receiver and transmitter ends of a link to ensure communications while minimizing power consumption.
2. Description of the Related Art
As noted in Wikipedia, forward error correction (FEC) or channel coding is a system of error control for data transmission, whereby the sender adds systematically generated redundant data to its messages, also known as an error-correcting code. A carefully designed redundancy permits the receiver to detect and correct a limited number of errors occurring anywhere in the message without the need to ask the sender for additional data. FEC gives the receiver an ability to correct errors without needing a reverse channel to request retransmission of data, but this advantage is at the cost of a fixed higher forward channel bandwidth. FEC is therefore applied in situations where retransmissions are relatively costly, or impossible such as when broadcasting to multiple receivers.
FEC processing in a receiver may be applied to a digital bit stream or in the demodulation of a digitally modulated carrier. For the latter, FEC is an integral part of the initial analog-to-digital conversion in the receiver. The Viterbi decoder implements a soft-decision algorithm to demodulate digital data from an analog signal corrupted by noise. Many FEC coders can also generate a bit-error rate (BER) signal which can be used as feedback to fine-tune the analog receiving electronics. The maximum fractions of errors or of missing bits that can be corrected is determined by the design of the FEC code, so different forward error correcting codes are suitable for different conditions.
The two main categories of FEC codes are block codes and convolutional codes. Block codes work on fixed-size blocks (packets) of bits or symbols of predetermined size. Practical block codes can generally be decoded in polynomial time to their block length. Convolutional codes work on bit or symbol streams of arbitrary length. They are most often decoded with the Viterbi algorithm, though other algorithms are sometimes used. Viterbi decoding allows asymptotically optimal decoding efficiency with increasing constraint length of the convolutional code, but at the expense of exponentially increasing complexity. A convolutional code can be turned into a block code, if desired, by “tail-biting”.
There are many types of block codes, but among the classical ones the most notable is Reed-Solomon coding because of its widespread use on the Compact disc, the DVD, and in hard disk drives. Golay, BCH, Multidimensional parity, and Hamming codes are other examples of classical block codes. FEC codes may be concatenated for improved performance. Many other related codes are well known in the art.
Signal throughput can also be enhanced using Physical Coding Sublayer (PCS). PCS typically helps to define physical layer specifications for networking protocols like Fast Ethernet, Gigabit Ethernet, and 10 Gigabit Ethernet. The Ethernet PCS sublayer is part of the Ethernet PHY layer (Layer 1). Besides some auto-negotiation, PCS performs 8 binary/10 binary (8B/10B) coding. 8B/10B is a line code that maps 8-bit symbols to 10-bit symbols to achieve dc voltage balance and bounded disparity. For 10 Gigabit Ethernet, PCS is associated with 64B/66B, which is a near dc-balanced waveform. When describing a periodic function in the frequency domain, the dc bias is the mean value of the waveform. A waveform with a zero dc component is known as a dc-balanced waveform. Bit errors can occur when a (relatively) long series of 1's create a dc level that charges the capacitor of the high-pass filter used as the AC coupler, bringing the signal input down incorrectly to a 0-level. In order to avoid these kinds of bit errors, most line codes are designed to produce dc-balanced, or near dc-balanced waveforms. The “cost” of dc balancing is the increase in signal overhead.
In processing electronic audio signals, pre-emphasis (PE) refers to a system process designed to increase, within a band of frequencies, the magnitude of some frequencies with respect to the magnitude of other frequencies in order to improve the overall signal-to-noise ratio—minimizing the adverse effects of such phenomena as attenuation distortion or saturation in subsequent parts of the transmitter or channel.
In high speed digital transmission, pre-emphasis is used to improve signal quality at the output of a data transmission. In transmitting signals at high data rates, the transmission medium may introduce distortions, so pre-emphasis is used to pre-distort the transmitted signal to correct for this distortion. When done properly this produces a received signal which more closely resembles the original or desired signal, producing fewer bit errors. At high data rates, pre-emphasis may include some modification in phase or group delay, as well as amplitude.
At the receiver side, equalization (EQ) can be used to enhance a received signal. Filters, attenuator, or amplifiers can be used to amplify or attenuation signal amplitude in portions of the pass band. Likewise, modifications can be performed upon signal phase and group delay to more accurately recover a signal that has been distorted through the transmission channel. These types of corrections may also be referred to as dispersion compensation.
In the case of electrical circuits, optical-to-electrical, or electrical-to-optical (E/O) conversion, signal enhancement can be often obtained by increasing the direct current (dc) supply voltage powering these circuits. For example, it is well known that CMOS circuitry operating speed improves with an increase in current. Likewise, optical-electrical conversion circuitry such as laser diodes and photodiodes typically perform better at higher dc supply voltages.
Thus, various types of electrical circuitry, optical circuitry, and processing algorithms can be used to improve signal quality or aid in signal recovery. In most cases, this circuitry is used to compensation for poor signal intensity or quality due to long links. Typically, there is no means other than manual intervention or open-loop “trial-and-error” to either turn off unneeded circuitry or to reduce the number of taps in an equalizer.
It would be advantageous if a process existed for modifying the signal enhancement mechanism needed to process a signal, so that a device could minimize power consumption by using only those mechanisms actually required to support communications.
It would be advantageous if the above-disclosed signal enhancement modifications could be performed on both the transmit and receive sides on the link.